Quantitative analysis and parametric imaging of 18F-labeled monomeric and dimeric RGD peptides using compartment model

A comparison study of dynamic [18F]Alfatide II imaging and [11C]MET in orthotopic rat models of glioblastoma

PURPOSE

To investigate and compare the dynamic positron emission tomography (PET) imaging with [18F]Alfatide II Imaging and [11C]Methionine ([11C]MET) in orthotopic rat models of glioblastoma multiforme (GBM), and to assess the utility of [18F]Alfatide II in detecting and evaluating neoangiogenesis in GBM.

METHODS

[18F]Alfatide II and [11C]MET were injected into the orthotopic GBM rat models (n = 20, C6 glioma cells), followed by dynamic PET/MR scans 21 days after surgery of tumor implantation. On the PET image with both radiotracers, the MRI-based volume-of-interest (VOI) was manually delineated encompassing glioblastoma. Time-activity curves were expressed as tumor-to-normal brain ratio (TNR) parameters and PET pharmacokinetic modeling (PKM) performed using 2-tissue-compartment models (2TCM). Immunofluorescent staining (IFS), western blotting and blocking experiment of tumor tissue were performed for the validation.

RESULTS

Compared to 11C-MET, [18F]Alfatide II presented a persistent accumulation in the tumor, albeit with a slightly lower SUVmean of 0.79 ± 0.25, and a reduced uptake in the contralateral normal brain tissue, respectively. This resulted in a markedly higher tumor-to-normal brain ratio (TNR) of 18.22 ± 1.91. The time-activity curve (TACs) showed a significant increase in radioactive uptake in tumor tissue, followed by a plateau phase up to 60 min for [18F]Alfatide II (time to peak:255 s) and 40 min for [11C]MET (time to peak:135 s) post injection. PKM confirmed significantly higher K1 (0.23/0.07) and K3 (0.26/0.09) in the tumor region compared to the normal brain with [18F]Alfatide II. Compared to [11C]MET imaging, PKM confirmed both significantly higher K1/K2 (1.24 ± 0.79/1.05 ± 0.39) and K3/K4 (11.93 ± 4.28/3.89 ± 1.29) in the tumor region with [18F]Alfatide II. IFS confirmed significant expression of integrin and tumor vascularization in tumor region.

CONCLUSION

[18F]Alfatide II demonstrates potential in imaging tumor-associated neovascularization in the context of glioblastoma multiforme (GBM), suggesting its utility as a tool for further exploration in neovascular characterization.

Evaluation of targeting αVβ3 in breast cancers using RGD peptide-based agents

Patients with HER2-positive and triple negative breast cancer (TNBC) are associated with increased risk to develop metastatic disease including reoccurring disease that is resistant to standard and targeted therapies. The αVβ3 has been implicated in BC including metastatic disease. The aims of this study were to investigate the potential of αVβ3-targeted peptides to deliver radioactive payloads to BC tumors expressing αVβ3 on the tumor cells or limited to the tumors' neovascular. Additionally, we aimed to assess the pharmacokinetic profile of the targeted α-particle therapy (TAT) agent [225Ac]Ac-DOTA-cRGDfK dimer peptide and the in vivo generated decay daughters. The expression of αVβ3 in a HER2-positive and a TNBC cell line were evaluated using western blot analysis. The pharmacokinetics of [111In]In-DOTA-cRGDfK dimer, a surrogate for the TAT-agent, was evaluated in subcutaneous mouse tumor models. The pharmacokinetic of the TAT-agent [225Ac]Ac-DOTA-cRGDfK dimer and its decay daughters were evaluated in healthy mice. Selective uptake of [111In]In-DOTA-cRGDfK dimer was shown in subcutaneous tumor models using αVβ3-positive tumor cells as well as αVβ3-negative tumor cells where the expression is limited to the neovasculature. Pharmacokinetic studies demonstrated rapid accumulation in the tumors with clearance from non-target organs. Dosimetric analysis of [225Ac]Ac-DOTA-cRGDfK dimer showed the highest radiation absorbed dose to the kidneys, which included the contributions from the free in vivo generated decay daughters. This study shows the potential of delivering radioactive payloads to BC tumors that have αVβ3 expression on the tumor cells as well as limited expression to the neovascular of the tumor. Furthermore, this work determines the radiation absorbed doses to normal organs/tissues and identified key organs that act as suppliers and receivers of the actinium-225 free in vivo generated α-particle-emitting decay daughters.

TSPO PET Imaging as a Potent Non-Invasive Biomarker for Diffuse Intrinsic Pontine Glioma in a Patient-Derived Orthotopic Rat Model

Diffuse intrinsic pontine gliomas (DIPG), the first cause of cerebral pediatric cancer death, will greatly benefit from specific and non-invasive biomarkers for patient follow-up and monitoring of drug efficacy. Since biopsies are challenging for brain tumors, molecular imaging may be a technique of choice to target and follow tumor evolution. So far, MR remains the imaging technique of reference for DIPG, although it often fails to define the extent of tumors, an essential parameter for therapeutic efficacy assessment. Thanks to its high sensitivity, positron emission tomography (PET) offers a unique way to target specific biomarkers in vivo. We demonstrated in a patient-derived orthotopic xenograft (PDOX) model in the rat that the translocator protein of 18 kDa (TSPO) may be a promising biomarker for monitoring DIPG tumors. We studied the distribution of 18F-DPA-714, a TSPO radioligand, in rats inoculated with HSJD-DIPG-007 cells. The primary DIPG human cell line HSJD-DIPG-007 highly represents this pediatric tumor, displaying the most prevalent DIPG mutations, H3F3A (K27M) and ACVR1 (R206H). Kinetic modeling and parametric imaging using the brain 18F-DPA-714 PET data enabled specific delineation of the DIPG tumor area, which is crucial for radiotherapy dose management.

The Feasibility of Targeted Magnetic Iron Oxide Nanoagent for Noninvasive IgA Nephropathy Diagnosis

IgA nephropathy is the most common glomerular disease in the world and has become a serious threat to human health. Accurate and non-invasive molecular imaging to detect and recognize the IgA nephropathy is critical for the subsequent timely treatment; otherwise, it may progress to end-stage renal disease and lead to glomerular dysfunction. In this study, we have developed a sensitive, specific, and biocompatible integrin αvβ3-targeted superparamagnetic Fe₃O₄ nanoparticles (NPs) for the noninvasive magnetic resonance imaging (MRI) of integrin αvβ3, which is overexpressed in glomerular mesangial region of IgA nephropathy. The rat model of IgA nephropathy was successfully established and verified by biochemical tests and histological staining. Meanwhile, the clinical ¹⁸F-AlF-NOTA-PRGD2 probe molecule was utilized to visualize and further confirmed the IgA nephropathy in vivo via positron emission computed tomography. Subsequently, the Fe₃O₄ NPs were conjugated with arginine–glycine–aspartic acid (RGD) molecules (Fe₃O₄-RGD), and their integrin αvβ3-targeted T2-weighted imaging (T2WI) potential has been carefully evaluated. The Fe₃O₄-RGD demonstrated great relaxation in vivo. The T2WI signal of renal layers in the targeted group at 3 h after intravenous injection of Fe₃O₄-RGD was distinctly lower than baseline, indicating MRI signal decreased in the established IgA nephropathy rat model. Moreover, the TEM characterization and Prussian blue staining confirmed that the Fe₃O₄-RGD was located at the region of glomerulus and tubular interstitium. Moreover, no obvious signal decreased was detected in the untargeted Fe₃O₄ treated and normal groups. Collectively, our results establish the possibility of Fe₃O₄-RGD serving as a feasible MRI agent for the noninvasive diagnosis of IgA nephropathy.

A comparison of the monomeric [68Ga]NODAGA-NGR and dimeric [68Ga]NOTA-(NGR)2 as aminopeptidase N ligand for positron emission tomography imaging in tumor-bearing mice

The aminopeptidase N (APN/CD13) is a key protein specifically expressed on activated endothelial cells and by various tumors, representing a promising target for molecular imaging and therapy of malignant diseases. It is known that the tripeptide NGR is a specific ligand for CD13, therefore radiolabeled NGR peptides are auspicious radiotracers for non-invasive imaging of CD13-positive tumors. From previous studies, it is known that the target affinity could be improved by molecules with multiple ligand sequences. Therefore, the aim of this study was to compare two NGR radioligands [⁶⁸Ga]NODAGA-NGR (NGR monomer) and [⁶⁸Ga]NOTA-(NGR)₂ (NGR dimer), the latter with two NGR ligand motifs, in vitro and in vivo. CD13 expression was determined by FACS in the human tumor cells A549, SKHep-1, and MDA-MB-231, followed by the investigation of the cell uptake of [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂. For in vivo evaluation of [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂, microPET and biodistribution were carried out in A549- and SKHep-1-bearing mice. After the final examination, tumors were cryo-conserved, cut, and stained against CD13 and CD31. A549 and SKHep-1 cells were identified as CD13 positive, whereas no CD13 expression was detected in MDA-MB-231 cells. The cell uptake study showed relatively low accumulation of both the NGR monomer and dimer in all tumor cell lines examined, with consistently higher cell uptake observed for the dimer than for the monomer. In vivo, [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂ accumulated in the tumors, with slightly higher tumor-to-muscle ratio for the NGR dimer in A549 and SKHep-1. The tumor-to-liver ratio of the NGR dimer was diminished in comparison to the NGR monomer. This finding was confirmed by biodistribution, which revealed higher accumulation in liver and spleen for the NGR dimer. Immunohistochemical staining confirmed the CD13 expression in the tumors and tumor-associated vessels. In conclusion, both the [⁶⁸Ga]NODAGA-NGR and the [⁶⁸Ga]NOTA-(NGR)₂ were found to be suitable for PET imaging of CD13-positive tumors. Despite slight differences in tumor-to-background ratio and organ accumulation, both radiotracers can be considered comparable.

Comparative evaluation of dimeric and monomeric forms of ADAPT scaffold protein for targeting of HER2-expressing tumours

ADAPTs are small engineered non-immunoglobulin scaffold proteins, which have demonstrated very promising features as vectors for radionuclide tumour targeting. Radionuclide imaging of human epidermal growth factor 2 (HER2) expression in vivo might be used for stratification of patients for HER2-targeting therapies. ADAPT6, which specifically binds to HER2, has earlier been shown to have very promising features for in vivo targeting of HER2 expressing tumours. In this study we tested the hypothesis that dimerization of ADAPT6 would increase the apparent affinity to HER2 and accordingly improve tumour targeting. To find an optimal molecular design of dimers, a series of ADAPT dimers with different linkers, -SSSG- (DiADAPT6L1), -(SSSG)₂- (DiADAPT6L2), and -(SSSG)₃- (DiADAPT6L3) was evaluated. Dimers in combination with optimal linker lengths demonstrated increased apparent affinity to HER2. The best variants, DiADAPT6L2 and DiADAPT6L3 were site-specifically labelled with ¹¹¹In and ¹²⁵I, and compared with a monomeric ADAPT6 in mice bearing HER2-expressing tumours. Despite higher affinity, both dimers had lower tumour uptake and lower tumour-to-organ ratios compared to the monomer. We conclude that improved affinity of a dimeric form of ADAPT does not compensate the disadvantage of increased size. Therefore, increase of affinity should be obtained by affinity maturation and not by dimerization.

Intravenous Administration-Oriented Pharmacokinetic Model for Dynamic Bioluminescence Imaging

OBJECTIVE

In vivo bioluminescence imaging (BLI) is a promising tool for monitoring the growth and metastasis of tumors. However, quantitative BLI research based on intravenous (IV) injection is limited, which greatly restricts its further application. To address this problem, we designed a pharmacokinetic (PK) model which is suitable for applying on IV administration of small amounts of D-Luciferin.

METHODS

After three weeks of postimplantation, mkn28-luc xenografted mice were subjected to 40-min dynamic BLI immediately following D-Luciferin intravenous injection on days 1, 3, 5, 7, and 9. Furthermore, the PK model was applied on dynamic BLI data to obtain the sum of kinetic rate constants (SKRC).

RESULTS

Results showed that the SKRC values decreased rapidly with the growth of the tumor. There was a statistical difference between the SKRC values measured at different time points, while the time point of luminous intensity peak was unaffected by the growth of the tumor.

CONCLUSION

In short, our results imply that dynamic BLI combined with our PK model can predict tumor growth earlier and with higher sensitivity compared to the conventional method, which is crucial for improving drug evaluation efficacy. In addition, the dynamic BLI may provide a valuable reference for the noninvasive acquiring arterial input function, which may also provide a new application prospect for hybrid PET-optical imaging.

The feasibility of 18F-AlF-NOTA-PRGD2 PET/CT for monitoring early response of Endostar antiangiogenic therapy in human nasopharyngeal carcinoma xenograft model compared with 18F-FDG

Purpose

Radiolabeled arginine-glycine-aspartic acid (RGD) peptides have been developed for PET imaging of integrin avβ3 in the tumor vasculature, leading to great potential for noninvasively evaluating tumor angiogenesis and monitoring antiangiogenic treatment. The aim of this study was to investigate a novel one-step labeled integrin-targeted tracer, ¹⁸F-AlF-NOTA-PRGD2, for PET/CT for detecting tumor angiogenesis and monitoring the early therapeutic efficacy of antiangiogenic agent Endostar in human nasopharyngeal carcinoma (NPC) xenograft model.

Experimental design and results

Mice bearing NPC underwent ¹⁸F-AlF-NOTA-PRGD2 PET/CT at baseline and after 2, 4, 7, and 14 days of consecutive treatment with Endostar or PBS, compared with ¹⁸F-FDG PET/CT. Tumors were harvested at all imaging time points for histopathological analysis with H & E and microvessel density (MVD) and integrin avβ3 immunostaining. The maximum percent injected dose per gram of body weight (%ID/gmax) tumor uptake of ¹⁸F-AlF-NOTA-PRGD2 PET/CT was significantly lower than that in the control group starting from day 2 (p < 0.01), much earlier and more accurately than that of ¹⁸F-FDG PET/CT. Moreover, a moderate linear correlation was observed between tumor MVD and the corresponding tumor uptake of ¹⁸F-AlF-NOTA-PRGD2 PET/CT (r = 0.853, p < 0.01).

Conclusions

¹⁸F-AlF-NOTA-PRGD2 PET/CT can be used for in vivo angiogenesis imaging and monitoring early response to Endostar antiangiogenic treatment in NPC xenograft model, favoring its potential clinical translation.

Investigation of injection dose and camera integration time on quantifying pharmacokinetics of a Cy5.5-GX1 probe with dynamic fluorescence imaging in vivo

The aim of this article is to investigate the influence of a tracer injection dose (ID) and camera integration time (IT) on quantifying pharmacokinetics of Cy5.5-GX1 in gastric cancer BGC-823 cell xenografted mice. Based on three factors, including whether or not to inject free GX1, the ID of Cy5.5-GX1, and the camera IT, 32 mice were randomly divided into eight groups and received 60-min dynamic fluorescence imaging. Gurfinkel exponential model (GEXPM) and Lammertsma simplified reference tissue model (SRTM) combined with a singular value decomposition analysis were used to quantitatively analyze the acquired dynamic fluorescent images. The binding potential (Bp) and the sum of the pharmacokinetic rate constants (SKRC) of Cy5.5-GX1 were determined by the SRTM and EXPM, respectively. In the tumor region, the SKRC value exhibited an obvious trend with change in the tracer ID, but the Bp value was not sensitive to it. Both the Bp and SKRC values were independent of the camera IT. In addition, the ratio of the tumor-to-muscle region was correlated with the camera IT but was independent of the tracer ID. Dynamic fluorescence imaging in conjunction with a kinetic analysis may provide more quantitative information than static fluorescence imaging, especially for a priori information on the optimal ID of targeted probes for individual therapy.

Evaluation of Tc-99 m Labeled Dimeric GX1 Peptides for Imaging of Colorectal Cancer Vasculature

PURPOSE

This study aimed to evaluate the potential of PEGylated dimeric GX1 peptide as a radiotracer for imaging of colorectal cancer vasculature in a LoVo tumor xenografted mouse model.

PROCEDURES

The [(99m)Tc]PEG-(GX1)2 peptide was synthesized and identified. Confocal immunofluorescence analysis, receptor binding assay, and competitive inhibition assay were performed to evaluate the binding specificity and the receptor binding affinity of PEG-(GX1)2 to Co-human umbilical vein endothelial cells (HUVECs). Single photon emission computed tomography imaging and biodistribution were performed to evaluate the targeting ability of PEG-(GX1)2 to colorectal cancer.

RESULTS

The studies in vitro suggested that PEG-(GX1)2 co-localized with Factor VIII in the perinuclear cytoplasm of Co-HUVECs and bound specifically to Co-HUVECs with a high affinity. The studies in vivo demonstrated that the targeting efficacy of PEG-(GX1)2 was superior to GX1.

CONCLUSIONS

PEGylation improved the affinity and the targeting ability of the GX1 peptide. PEG-(GX1)2 is a more promising probe for imaging of colorectal vasculature than GX1.

MR Imaging Biomarkers to Monitor Early Response to Hypoxia-Activated Prodrug TH-302 in Pancreatic Cancer Xenografts

TH-302 is a hypoxia-activated prodrug known to activate selectively under the hypoxic conditions commonly found in solid tumors. It is currently being evaluated in clinical trials, including two trials in Pancreatic Ductal Adenocarcinomas (PDAC). The current study was undertaken to evaluate imaging biomarkers for prediction and response monitoring of TH-302 efficacy in xenograft models of PDAC. Dynamic contrast-enhanced (DCE) and diffusion weighted (DW) magnetic resonance imaging (MRI) were used to monitor acute effects on tumor vasculature and cellularity, respectively. Three human PDAC xenografts with known differential responses to TH-302 were imaged prior to, and at 24 h and 48 hours following a single dose of TH-302 or vehicle to determine if imaging changes presaged changes in tumor volumes. DW-MRI was performed at five b-values to generate apparent diffusion coefficient of water (ADC) maps. For DCE-MRI, a standard clinically available contrast reagent, Gd-DTPA, was used to determine blood flow into the tumor region of interest. TH-302 induced a dramatic decrease in the DCE transfer constant (K^trans) within 48 hours after treatment in the sensitive tumors, Hs766t and Mia PaCa-2, whereas TH-302 had no effect on the perfusion behavior of resistant SU.86.86 tumors. Tumor cellularity, estimated from ADC, was significantly increased 24 and 48 hours after treatment in Hs766t, but was not observed in the Mia PaCa-2 and SU.86.86 groups. Notably, growth inhibition of Hs766t was observed immediately (day 3) following initiation of treatment, but was not observed in MiaPaCa-2 tumors until 8 days after initiation of treatment. Based on these preclinical findings, DCE-MRI measures of vascular perfusion dynamics and ADC measures of cell density are suggested as potential TH-302 response biomarkers in clinical trials.

Clinical Application of Radiolabeled RGD Peptides for PET Imaging of Integrin αvβ3

Molecular imaging for non-invasive assessment of angiogenesisis is of great interest for clinicians because of the wide-spread application of anti-angiogenic cancer therapeutics. Besides, many other interventions that involve the change of blood vessel/tumor microenvironment would also benefit from such imaging strategies. Of the imaging techniques that target angiogenesis, radiolabeled Arg-Gly-Asp (RGD) peptides have been a major focus because of their high affinity and selectivity for integrin α_vβ₃--one of the most extensively examined target of angiogenesis. Since the level of integrin α_vβ₃ expression has been established as a surrogate marker of angiogenic activity, imaging α_vβ₃ expression can potentially be used as an early indicator of effectiveness of antiangiogenic therapy at the molecular level. In this review, we summarize RGD-based PET tracers that have already been used in clinical trials and intercompared them in terms of radiosynthesis, dosimetry, pharmacokinetics and clinical applications. A perspective of their future use in the clinic is also provided.

(18)F-Alfatide II PET/CT in healthy human volunteers and patients with brain metastases

PURPOSE

We report the biodistribution and radiation dosimetry of an integrin αvβ3 specific PET tracer (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2) (denoted as (18)F-Alfatide II). We also assessed the value of (18)F-Alfatide II in patients with brain metastases.

METHODS

A series of torso (from the skull to the thigh) static images were acquired in five healthy volunteers (3 M, 2 F) at 5, 10, 15, 30, 45, and 60 min after injection of (18)F-Alfatide II (257 ± 48 MBq). Regions of interest (ROIs) were drawn manually, and the time-activity curves (TACs) were obtained for major organs. Nine patients with brain metastases were examined by static PET imaging with (18)F-FDG (5.55 MBq/kg) and (18)F-Alfatide II.

RESULTS

Injection of (18)F-Alfatide II was well tolerated in all healthy volunteers, with no serious tracer-related adverse events found. (18)F-Alfatide II showed rapid clearance from the blood pool and kidneys. The total effective dose equivalent (EDE) and effective dose (ED) were 0.0277 ± 0.003 mSv/MBq and 0.0198 ± 0.002 mSv/MBq, respectively. The organs with the highest absorbed dose were the kidneys and the spleen. Nine patients with 20 brain metastatic lesions identified by MRI and/or CT were enrolled in this study. All 20 brain lesions were visualized by (18)F-Alfatide II PET, while only ten lesions were visualized by (18)F-FDG, and 13 by CT.

CONCLUSION

F-Alfatide II is a safe PET tracer with a favorable dosimetry profile. The observed ED suggests that (18)F-Alfatide II is feasible for human studies. (18)F-Alfatide II has potential value in finding brain metastases of different cancers as a biomarker of angiogenesis.

Biodistribution of the ¹⁸F-FPPRGD₂ PET radiopharmaceutical in cancer patients: an atlas of SUV measurements

PURPOSE

The aim of this study was to investigate the biodistribution of 2-fluoropropionyl-labeled PEGylated dimeric arginine-glycine-aspartic acid (RGD) peptide (PEG3-E[c{RGDyk}]2) ((18)F-FPPRGD2) in cancer patients and to compare its uptake in malignant lesions with (18)F-FDG uptake.

METHODS

A total of 35 patients (11 men, 24 women, mean age 52.1 ± 10.8 years) were enrolled prospectively and had (18)F-FPPRGD2 PET/CT prior to treatment. Maximum standardized uptake values (SUVmax) and mean SUV (SUVmean) were measured in 23 normal tissues in each patient, as well as in known or suspected cancer lesions. Differences between (18)F-FPPRGD2 uptake and (18)F-FDG uptake were also evaluated in 28 of the 35 patients.

RESULTS

Areas of high (18)F-FPPRGD2 accumulation (SUVmax range 8.9 - 94.4, SUVmean range 7.1 - 64.4) included the bladder and kidneys. Moderate uptake (SUVmax range 2.1 - 6.3, SUVmean range 1.1 - 4.5) was found in the choroid plexus, salivary glands, thyroid, liver, spleen, pancreas, small bowel and skeleton. Compared with (18)F-FDG, (18)F-FPPRGD2 showed higher tumor-to-background ratio in brain lesions (13.4 ± 8.5 vs. 1.1 ± 0.5, P < 0.001), but no significant difference in body lesions (3.2 ± 1.9 vs. 4.4 ± 4.2, P = 0.10). There was no significant correlation between the uptake values (SUVmax and SUVmean) for (18)F FPPRGD2 and those for (18)F-FDG.

CONCLUSION

The biodistribution of (18)F-FPPRGD2 in cancer patients is similar to that of other RGD dimer peptides and it is suitable for clinical use. The lack of significant correlation between (18)F-FPPRGD2 and (18)F-FDG uptake confirms that the information provided by each PET tracer is different.

PET radiopharmaceuticals for imaging integrin expression: tracers in clinical studies and recent developments

Noninvasive determination of integrin expression has become an interesting approach in nuclear medicine. Since the discovery of the first 18F-labeled cyclic RGD peptide as radiotracer for imaging integrin αvβ3 expression in vivo, there have been carried out enormous efforts to develop RGD peptides for PET imaging. Moreover, in recent years, additional integrins, including α5β1 and αvβ6 came into the focus of pharmaceutical radiochemistry. This review will discuss the tracers already evaluated in clinical trials and summarize the preliminary outcome. It will also give an overview on recent developments to further optimize the first-generation compounds such as [18F]Galacto-RGD. This includes recently developed 18F-labeling strategies and also new approaches in 68Ga-complex chemistry. Furthermore, the approaches to develop radiopharmaceuticals targeting integrin α5β1 and αvβ6 will be summarized and discussed.

(18)F-alfatide II and (18)F-FDG dual-tracer dynamic PET for parametric, early prediction of tumor response to therapy

A single dynamic PET acquisition using multiple tracers administered closely in time could provide valuable complementary information about a tumor's status under quasiconstant conditions. This study aimed to investigate the utility of dual-tracer dynamic PET imaging with (18)F-alfatide II ((18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2) and (18)F-FDG for parametric monitoring of tumor responses to therapy.

METHODS

We administered doxorubicin to one group of athymic nude mice with U87MG tumors and paclitaxel protein-bound particles to another group of mice with MDA-MB-435 tumors. To monitor therapeutic responses, we performed dual-tracer dynamic imaging, in sessions that lasted 90 min, starting with injection via the tail vein catheters with (18)F-alfatide II, followed 40 min later by (18)F-FDG. To achieve signal separation of the 2 tracers, we fit a 3-compartment reversible model to the time-activity curve of (18)F-alfatide II for the 40 min before (18)F-FDG injection and then extrapolated to 90 min. The (18)F-FDG tumor time-activity curve was isolated from the 90-min dual-tracer tumor time-activity curve by subtracting the fitted (18)F-alfatide II tumor time-activity curve. With separated tumor time-activity curves, the (18)F-alfatide II binding potential (Bp = k3/k4) and volume of distribution (VD) and (18)F-FDG influx rate ((K1 × k3)/(k2 + k3)) based on the Patlak method were calculated to validate the signal recovery in a comparison with 60-min single-tracer imaging and to monitor therapeutic response.

RESULTS

The transport and binding rate parameters K1-k3 of (18)F-alfatide II, calculated from the first 40 min of the dual-tracer dynamic scan, as well as Bp and VD correlated well with the parameters from the 60-min single-tracer scan (R(2) > 0.95). Compared with the results of single-tracer PET imaging, (18)F-FDG tumor uptake and influx were recovered well from dual-tracer imaging. On doxorubicin treatment, whereas no significant changes in static tracer uptake values of (18)F-alfatide II or (18)F-FDG were observed, both (18)F-alfatide II Bp and (18)F-FDG influx from kinetic analysis in tumors showed significant decreases. For therapy of MDA-MB-435 tumors with paclitaxel protein-bound particles, a significant decrease was observed only with (18)F-alfatide II Bp value from kinetic analysis but not (18)F-FDG influx.

CONCLUSION

The parameters fitted with compartmental modeling from the dual-tracer dynamic imaging are consistent with those from single-tracer imaging, substantiating the feasibility of this methodology. Even though no significant differences in tumor size were found until 5 d after doxorubicin treatment started, at day 3 there were already substantial differences in (18)F-alfatide II Bp and (18)F-FDG influx rate. Dual-tracer imaging can measure (18)F-alfatide II Bp value and (18)F-FDG influx simultaneously to evaluate tumor angiogenesis and metabolism. Such changes are known to precede anatomic changes, and thus parametric imaging may offer the promise of early prediction of therapy response.

First experience of 18F-alfatide in lung cancer patients using a new lyophilized kit for rapid radiofluorination

(18)F-FPPRGD2, which was approved for clinical study recently, has favorable properties for integrin targeting and showed potential for antiangiogenic therapy and early response monitoring. However, the time-consuming multiple-step synthesis may limit its widespread applications in the clinic. In this study, we developed a simple lyophilized kit for labeling PRGD2 peptide ((18)F-AlF-NOTA-PRGD2, denoted as (18)F-alfatide) using a fluoride-aluminum complex that significantly simplified the labeling procedure.

METHODS

Nine patients with a primary diagnosis of lung cancer were examined by both static and dynamic PET imaging with (18)F-alfatide, and 1 tuberculosis patient was investigated using both (18)F-alfatide and (18)F-FDG imaging. Standardized uptake values were measured in tumors and other main organs at 30 min and 1 h after injection. Kinetic parameters were calculated by Logan graphical analysis. Immunohistochemistry and staining intensity quantification were performed to confirm the expression of integrin α(v)β(3).

RESULTS

Under the optimal conditions, the whole radiosynthesis including purification was accomplished within 20 min with a decay-corrected yield of 42.1% ± 2.0% and radiochemical purity of more than 95%. (18)F-alfatide PET imaging identified all tumors, with mean standardized uptake values of 2.90 ± 0.10. Tumor-to-muscle and tumor-to-blood ratios were 5.87 ± 2.02 and 2.71 ± 0.92, respectively.

CONCLUSION

(18)F-alfatide can be produced with excellent radiochemical yield and purity via a simple, 1-step, lyophilized kit. PET scanning with (18)F-alfatide allows specific imaging of αvβ3 expression with good contrast in lung cancer patients. This technique might be used for the assessment of angiogenesis and for planning and response evaluation of cancer therapies that would affect angiogenesis status and integrin expression levels.

Nanotheranostics for personalized medicine

Nanotheranostics, the integration of diagnostic and therapeutic function in one system using the benefits of nanotechnology, is extremely attractive for personalized medicine. Because treating cancer is not a one-size-fits-all scenario, it requires therapy to be adapted to the patient's specific biomolecules. Personalized and precision medicine (PM) does just that. It identifies biomarkers to gain an understanding of the diagnosis and in turn treating the specific disorder based on the precise diagnosis. By predominantly utilizing the unique properties of nanoparticles to achieve biomarker identification and drug delivery, nanotheranostics can be applied to noninvasively discover and target image biomarkers and further deliver treatment based on the biomarker distribution. This is a large and hopeful role theranostics must fill. However, as described in this expert opinion, current nanotechnology-based theranostics systems engineered for PM applications are not yet sufficient. PM is an ever-growing field that will be a driving force for future discoveries in biomedicine, especially cancer theranostics. In this article, the authors dissect the requirements for successful nanotheranostics-based PM.

Dynamic PET and Optical Imaging and Compartment Modeling using a Dual-labeled Cyclic RGD Peptide Probe

Purpose: The aim of this study is to determine if dynamic optical imaging could provide comparable kinetic parameters to that of dynamic PET imaging by a near-infrared dye/⁶⁴Cu dual-labeled cyclic RGD peptide.

Methods: The integrin α_vβ₃ binding RGD peptide was conjugated with a macrocyclic chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for copper labeling and PET imaging and a near-infrared dye ZW-1 for optical imaging. The in vitro biological activity of RGD-C(DOTA)-ZW-1 was characterized by cell staining and receptor binding assay. Sixty-min dynamic PET and optical imaging were acquired on a MDA-MB-435 tumor model. Singular value decomposition (SVD) method was applied to compute the dynamic optical signal from the two-dimensional optical projection images. Compartment models were used to quantitatively analyze and compare the dynamic optical and PET data.

Results: The dual-labeled probe ⁶⁴Cu-RGD-C(DOTA)-ZW-1 showed integrin specific binding in vitro and in vivo. The binding potential (Bp) derived from dynamic optical imaging (1.762 ± 0.020) is comparable to that from dynamic PET (1.752 ± 0.026).

Conclusion: The signal un-mixing process using SVD improved the accuracy of kinetic modeling of 2D dynamic optical data. Our results demonstrate that 2D dynamic optical imaging with SVD analysis could achieve comparable quantitative results as dynamic PET imaging in preclinical xenograft models.

Quantitative analysis and comparison study of [18F]AlF-NOTA-PRGD2, [18F]FPPRGD2 and [68Ga]Ga-NOTA-PRGD2 using a reference tissue model

With favorable pharmacokinetics and binding affinity for α_vβ₃ integrin, ¹⁸F-labeled dimeric cyclic RGD peptide ([¹⁸F]FPPRGD2) has been intensively used as a PET imaging probe for lesion detection and therapy response monitoring. A recently introduced kit formulation method, which uses an ¹⁸F-fluoride-aluminum complex labeled RGD tracer ([¹⁸F]AlF-NOTA-PRGD2), provides a strategy for simplifying the labeling procedure to facilitate clinical translation. Meanwhile, an easy-to-prepare ⁶⁸Ga-labeled NOTA-PRGD2 has also been reported to have promising properties for imaging integrin α_vβ₃. The purpose of this study is to quantitatively compare the pharmacokinetic parameters of [¹⁸F]FPPRGD2, [¹⁸F]AlF-NOTA-PRGD2, and [⁶⁸Ga]Ga-NOTA-PRGD2. U87MG tumor-bearing mice underwent 60-min dynamic PET scans following the injection of three tracers. Kinetic parameters were calculated using Logan graphical analysis with reference tissue. Parametric maps were generated using voxel-level modeling. All three compounds showed high binding potential (Bp_ND = k₃/k₄) in tumor voxels. [¹⁸F]AlF-NOTA-PRGD2 showed comparable Bp_ND value (3.75±0.65) with those of [¹⁸F]FPPRGD2 (3.39±0.84) and [⁶⁸Ga]Ga-NOTA-PRGD2 (3.09±0.21) (p>0.05). Little difference was found in volume of distribution (V_T) among these three RGD tracers in tumor, liver and muscle. Parametric maps showed similar kinetic parameters for all three tracers. We also demonstrated that the impact of non-specific binding could be eliminated in the kinetic analysis. Consequently, kinetic parameter estimation showed more comparable results among groups than static image analysis. In conclusion, [¹⁸F]AlF-NOTA-PRGD2 and [⁶⁸Ga]Ga-NOTA-PRGD2 have comparable pharmacokinetics and quantitative parameters compared to those of [¹⁸F]FPPRGD2. Despite the apparent difference in tumor uptake (%ID/g determined from static images) and clearance pattern, the actual specific binding component extrapolated from kinetic modeling appears to be comparable for all three dimeric RGD tracers.